1. Field of the Invention
The present invention relates to an electron shower apparatus provided as part of an ion implanting apparatus used for the production of semiconductors. The electron shower apparatus is used to prevent electrostatic discharge damage to insulating parts in a sample plate.
2. Description of the Background Art
FIG. 17 schematically shows a conventional ion implanting (irradiating) apparatus which is an ion beam irradiating apparatus disclosed in Japanese Patent Application No. 2-79628 by the applicant of the present invention. Reference numeral 1 designates an electron shower apparatus mounted to a Faraday cup 7; 2 designates a thermoelectronic emission source; 3 designates an electron extracting electrode; 20 designates an opening of the electron extracting electrode 3; 4 designates a decelerating electrode; .21 designates an opening of the decelerating electrode 4; 6 designates a power source for heating the thermoelectronic emission source 2; 8 designates a sample table; 9 designates a sample; 10 designates an ion beam; and 19 designates a power source for applying energy toward the sample 9 to electrons.
FIG. 18 illustrates the movement of an electron emitted from the thermoelectronic emission source 2 of FIG. 17. Reference numeral 22 designates equipotential surfaces formed between the extracting electrode 3 and decelerating electrode 4.
Operations will be described with reference to FIGS. 17 and 18. A decelerating electric field formed between the electron extracting electrode 3 and decelerating electrode 4 in the electron shower apparatus 1 of FIG. 17 is represented by the equipotential surfaces 22 of FIG. 18. An electron e.sup.- 11 extracted from the thermoelectronic emission source 2 by the extracting electrode 3 enters the decelerating electric field perpendicularly to the axis of the ion beam 10. Since the electron 11 is decelerated perpendicularly to the equipotential surfaces 22 in the decelerating electric field, an emitting angle .alpha. of the electron 11 is expressed as: ##EQU1## where .theta. is an inclination angle of the equipotential surfaces 22 in the vicinity of the openings 20, 21 of the extracting and decelerating electrodes 3, 4 to the axis of the ion beam 10; Va is an extraction voltage; and Vd is a deceleration voltage.
The electron emitted from the decelerating electrode 4 and applied to the ion beam 10 has a component of velocity toward the sample 9. The generation of a slight potential at the power source 19 for applying energy toward the sample 9 to the electron enables the electron to travel toward the sample 9 without difficulty.
Relatively low energy electrons (primary electrons) are thus transported directly toward the sample, whereby the sample 9 charged positive by ions is neutralized while the sample 9 is prevented from being charged negative by the electrons.
Such a conventional process, however, has a problem in that a magnetic field generated by a heating current traps the low energy electrons so that the electrons in sufficient quantity to neutralize the positive charged sample 9 are not transported to the sample 9. The reason is discussed below. When the thermoelectronic emission source 2 is a bar-like tungsten filament of 1 mm in diameter, a filament heating current of 40 A or more in DC is required. A magnetic field B.sub.0 generated by the filament heating current outside of the filament is expressed as: EQU B.sub.0 (r)=.mu..sub.0 I/(2.pi.r) (2)
where .mu..sub.0 is a magnetic permeability in a vacuum; I is a filament current; and r is a distance from the center of the filament.
The magnetic fields calculated from Equation 2 are 160 gauss and 40 gauss in the positions where I=40 A and r =0.5 mm and where I=40 A and r=2 mm, respectively.
The Larmor radius r.sub.L of the electron having energy of E eV in the position where the magnetic field is B gauss is expressed as: ##EQU2## where m is the mass of electron; and e is an elementary electric charge.
The Larmor radius is approximately 15 mm where the magnetic field is 40 gauss and electronic energy is 300 eV, and the Larmor radius is approximately 5 mm where the magnetic field is 40 gauss and electronic energy is 30 eV. The electronic energy of 300 eV is primary electronic energy frequently used in an electron shower of the type in which secondary electrons are utilized, and the electronic energy of 30 eV is primary electronic energy to be used in an electron shower of the type in which primary low energy electrons are utilized.
The Larmor radius of 15 mm at the energy of 300 eV is three to eight times larger than the spacing of 2 to 5 mm between the electrodes which is adopted in the practical electron shower, and the Larmor radius of 5 mm at the energy of 30 eV is only as approximately large as the spacing to a little over twice larger than the spacing. This provides the indication that the electron having the energy of 30 eV (low energy) is more susceptible to the magnetic field than the electron having the energy of 300 eV. When the primary electron having low energy (e.g., 30 eV or less) is used, an electron orbit is largely bent by the magnetic field, so that the electron strikes the electrodes. The electron is not extracted efficiently or the orbit of the extracted electron is largely bent. As a result, it is considered that the electrons in sufficient quantity are not transported to the target sample.
In the electron shower apparatus of the prior art, in practice, a current of several tens mA passes through the decelerating electrode opening 21 where the extraction voltage is 300 V and the deceleration voltage is 300 V (without deceleration), and a current of only 1 mA or less passes through the opening 21 where the extraction voltage is 300 V and the deceleration voltage is 30 V. However when the filament heating current is changed to an alternating current (sine wave), a peak of the current of approximately 20 mA is observed at the point where the filament heating current is zero when the extraction voltage is 300 V and the deceleration voltage is 30 V. It is apparent from these facts that the magnetic field generated by the filament current exerts adverse effect on the low energy electrons.
The quantitative result of the influence of the magnetic field generated by the filament current on neutralizing electrons is easily derived from an equation of motion of charged particles and the like. Detailed description thereof will be omitted herein.
Decrease in cross-sectional area of the filament 2 to increase the resistance of the filament 2 enable the filament heating current to decrease. This process, however, has disadvantages. One of the disadvantages is that the thin filament 2 is prone to thermal deformation and short lifetime. This is a demerit in long-time, stable, continuous drive of the electron shower apparatus 1. The other disadvantage is a large potential difference across the filament 2 because of a large proportion of increase in resistance compared with the proportion of decrease in current. This means large scatter of electron energies. This disadvantage prevents the electrons from going low energy and causes deviation of electron distribution. The filament 2, when shortened, prevents the disadvantage, but must be prepared in plurality for ensuring a shower current, resulting in a complicated structure.